#include <iostream>
#include <cstdio>
using namespace std;

const int maxn =  20002;

#define F(x) ((x)/3+((x)%3==1?0:tb))
#define G(x) ((x)<tb?(x)*3+1:((x)-tb)*3+2)

int wa[maxn],wb[maxn],wv[maxn],wss[maxn];

inline int c0(int *r,int a,int b){
    return r[a]==r[b]&&r[a+1]==r[b+1]&&r[a+2]==r[b+2];
}

inline int c12(int k,int *r,int a,int b){
    if(k==2)
        return r[a]<r[b]||r[a]==r[b]&&c12(1,r,a+1,b+1);
    else
        return r[a]<r[b]||r[a]==r[b]&&wv[a+1]<wv[b+1];
}

void sort(int *r,int *a,int *b,int n,int m){
     int i;
     for(i=0;i<n;i++) wv[i]=r[a[i]];
     for(i=0;i<m;i++) wss[i]=0;
     for(i=0;i<n;i++) wss[wv[i]]++;
     for(i=1;i<m;i++) wss[i]+=wss[i-1];
     for(i=n-1;i>=0;i--) b[--wss[wv[i]]]=a[i];
     return;
}

void dc3(int *r,int *sa,int n,int m){
     int i,j,*rn=r+n,*san=sa+n,ta=0,tb=(n+1)/3,tbc=0,p;
     r[n]=r[n+1]=0;

     for(i=0;i<n;i++)
         if(i%3!=0) wa[tbc++]=i;

     sort(r+2,wa,wb,tbc,m);
     sort(r+1,wb,wa,tbc,m);
     sort(r,wa,wb,tbc,m);

     for(p=1,rn[F(wb[0])]=0,i=1;i<tbc;i++)
        rn[F(wb[i])]=c0(r,wb[i-1],wb[i])?p-1:p++;

     if(p<tbc)
         dc3(rn,san,tbc,p);
     else
         for(i=0;i<tbc;i++)
             san[rn[i]]=i;

     for(i=0;i<tbc;i++)
         if(san[i]<tb)
             wb[ta++]=san[i]*3;

     if(n%3==1)
         wb[ta++]=n-1;

     sort(r,wb,wa,ta,m);

     for(i=0;i<tbc;i++)
         wv[wb[i]=G(san[i])]=i;

     for(i=0,j=0,p=0;i<ta && j<tbc;p++)
        sa[p]=c12(wb[j]%3,r,wa[i],wb[j])?wa[i++]:wb[j++];

     for(;i<ta;p++)
         sa[p]=wa[i++];

     for(;j<tbc;p++)
         sa[p]=wb[j++];

     return;
}

int rank[maxn],height[maxn];
void calheight(int *r,int *sa,int n){
     int i,j,k=0;
     for(i=1;i<=n;i++)
         rank[sa[i]]=i;

     for(i=0;i<n;height[rank[i++]]=k)
        for(k?k--:0,j=sa[rank[i]-1];r[i+k]==r[j+k];k++);
     return;
}

int check(int *sa,int n,int k){
    int i,max=sa[1],min=sa[1];
    for(i=2;i<=n;i++){
      if(height[i]<k)
          max=min=sa[i];
      else{
        if(sa[i]<min) min=sa[i];
        if(sa[i]>max) max=sa[i];
        if(max-min>k) return(1);
      }
    }
    return(0);
}

int r[maxn*3],sa[maxn*3];
int main(){
    int i,j=0,k,n;
    int min,mid,max;
    scanf("%d",&n);
    while(n!=0){
      n--;
      scanf("%d",&j);
      for(i=0;i<n;i++){
        scanf("%d",&k);
        r[i]=k-j+100;
        j=k;
      }
      r[n]=0;
      dc3(r,sa,n+1,200);
      calheight(r,sa,n);
      min=1;max=n/2;
      while(min<=max){
        mid=(min+max)/2;
        if(check(sa,n,mid)) min=mid+1;
        else max=mid-1;
      }
      if(max>=4) printf("%d\n",max+1);
      else printf("0\n");
      scanf("%d",&n);
    }
    return 0;
}
